Transluminal coronary angioplasty involves the nonsurgical widening of a passage through an artery that has been narrowed or stenosed by deposits of plaque or plaque-ridden tissue. In one widely used approach, an inflatable balloon mounted on a catheter enlarges the passage through the deposit. In other known approaches the deposit is vaporized with laser energy transmitted through a catheter, or a passage is enlarged by plowing through the deposit with a hot tip device mounted at the distal end of a catheter. Regardless of the technique used for widening the passage through the obstruction, it is desirable for the cardiologist to measure the vessel cross-section before and after the angioplasty procedure in order to evaluate its effectiveness.
A device for measuring the cross sectional area of a blood vessel is disclosed in U.S. Pat. No. 3,896,373 issued July 22, 1975 to Zelby. Two electrodes that are spaced apart by a predetermined distance are secured to the outer surface of a catheter tube, and conductors extend from the electrodes through the tube to the proximal end of the catheter. The catheter carrying the electrodes is advanced through the blood vessel to a measurement site, and an AC voltage is applied to the electrodes. The voltage drop across the electrodes is indicative of the cross-sectional area of the blood vessel between the electrodes, since the applied voltage produces a current through the blood in the vessel.
In order to use the Zelby device in conjunction with an angioplasty procedure, the device is first advanced to the site of the obstruction to perform a measurement of vessel cross-section. Then the measurement device is withdrawn, and the balloon catheter is advanced to the obstructed site in order to perform the dilatation. Then the balloon catheter is withdrawn, and the measurement device is again advanced to the site to perform a second measurement of vessel cross-section. Since both the measurement device and the dilatation catheter can be difficult to advance to the obstructed site, the entire procedure is time consuming and is traumatic to the patient.
A dimension-sensitive angioplasty catheter having an inflatable balloon and a plurality of vessel-measuring electrodes near its distal end is disclosed in U.S. Pat. No. 4,587,985 issued May 13, 1986 to Salo et al. Each of the electrodes is mounted on the surface of the catheter tube and is individually connected to the proximal end of the catheter. One pair of electrodes is selected for connection to the output of an oscillator, and a second pair of electrodes is selected for sensing a signal that results from conduction through the blood in the vessel. While the Salo et al catheter avoids the problem of separate devices for vessel measurement and dilatation, the complexity of the device makes it difficult to fabricate with a sufficiently small diameter and sufficient flexibility for use in transluminal coronary angioplasty. In addition to the stiffness added by multiple electrodes, a separate conductor for each electrode passes through the catheter shaft.
Because of the requirement for accessing blood vessels of very small diameter, it has become commonplace in transluminal coronary angioplasty to use guidewires for controlling the placement of catheters. Catheters of sufficiently small diameter to be used in a small blood vessel typically lack the torsional rigidity to be adequately controlled as they are advanced through the vascular system to the obstructed site. Guidewires have an extremely small diameter, flexibility and sufficient torsional rigidity to be advanced to very small diameter blood vessels. The catheter is then advanced over the guidewire to the obstructed site. A steerable guidewire suitable for use in a balloon dilatation procedure is disclosed in U.S. Pat. No. 4,545,390 issued Oct. 8, 1985 to Leary and assigned to the assignee of the present application. The guidewire includes a small diameter, flexible rod having a distal region which is tapered. The tapered distal portion of the rod is surrounded by a helically-wound spring. The tip region of the guidewire is very flexible and can be bent to a predetermined shape which assists in guiding the device to an obstructed site. The guidewire disclosed in the Leary patent has no measurement capability.
A catheter including an intra-aortic balloon and a stylet normally used for twisting and untwisting the balloon is disclosed in U.S. Pat. No. 4,552,127, issued Nov. 12, 1985 to Schiff. An EKG electrode is affixed to the distal end of the stylet, and the stylet provides an electrical path from the EKG electrode to the proximal end of the catheter. A percutaneous lead that can be used for endocardial functions including mapping, ablation and pacing is disclosed in U.S. Pat. No. 4,660,571, issued Apr. 28, 1987 to Hess et al. In the Hess et al patent, a shaft utilized for torque control has an electrode at its distal end. The shaft functions as an electrical conductor from the electrode to the proximal end of the lead. An endocardial lead having a pair of spaced-apart, helically-wound electrodes on the outer surface of a flexible tube is disclosed in U.S. Pat. No. 4,481,953, issued Nov. 13, 1984 to Gold et al. A catheter for measuring blood flow including a pair of spaced-apart, helically-wound electrodes on the outer surface of a catheter tube is disclosed in U.S. Pat. No. 3,377,037, issued Nov. 20, 1973 to Kolin.
It is a general object of the present invention to provide a device for measuring the cross-sectional area of a blood vessel while avoiding the aforementioned problems.
It is another object of the present invention to provide improved methods and apparatus for measuring the cross-sectional area of a blood vessel and for measuring blood flow rate.
It is a further object of the present invention to provide a steerable guidewire having electrodes for measuring the cross-sectional area of a blood vessel and for measuring blood flow rate.
It is a further object of the present invention to provide a small diameter, steerable guidewire having a pair of spaced-apart electrodes near its distal end.
It is yet another object of the present invention to provide a small diameter, steerable guidewire having a highly flexible tip portion provided with a pair of spaced-apart electrodes.